1. Field of Invention
The present invention pertains to the field of demodulators. More particularly, this invention relates to controlling an adaptive equalizer in a demodulator.
2. Art Background
Communication systems commonly employ signal modulation to generate an information signal which is suitable for transmission via a physical communication path. For example, signal modulation is commonly employed in communication systems that transport information signals via transmission lines. Such communication systems include cable networks.
Such a communication system usually includes a transmitting station having a modulator that generates the information signal and a receiving station having a demodulator that extracts the information carried by the information signal. Typically, the transmitting and receiving stations are interconnected via one or more physical communication paths.
The communication paths in a typical communication system commonly include a variety of elements which introduce imperfections into the information signal. For example, transmission line connectors commonly cause signal reflections that distort the information signal. In addition, components such as signal amplifiers and filters may distort the information signal. Moreover, communication paths commonly have non-linear frequency and phase response which introduces further distortions into an information signal.
Prior demodulators may include an adaptive equalizer which is intended to compensate for the distortions which may be introduced into the information signal during transmission. A typical adaptive equalizer includes a filter and circuitry that continually adapts the filter according to a particular adaptation method. Prior adaptation methods are usually based upon a determination of an error measure for the adaptive equalizer. The adaptive equalizer is said to reach convergence when this error measure is small enough to yield a reliable output signal for the demodulator.
An adaptive equalizer may be classified as either blind or non-blind. A non-blind adaptive equalizer may be defined as an equalizer that adapts to a training sequence of symbols which is periodically carried in the information signal. A blind adaptive equalizer may be defined as an equalizer that adapts to a random symbol sequence in the information signal without the aid of a training sequence. It may in some systems be desirable to employ a blind adaptive equalizer. For example, a blind adaptive equalizer would eliminate the need for a training sequence which would ordinarily decrease the information throughput of a communication system.
Unfortunately, prior adaptation methods which may be useful for a blind adaptive equalizer commonly have difficulty in reaching a desirable point of convergence. For example, a common prior adaptation method in a blind adaptive equalizer employs a constant modulus algorithm (CMA) to recursively determine an error measure. Typically, the CMA adaptation method initially moves an equalizer toward convergence. The CMA adaptation method, however, usually has difficulty reaching a point of convergence that will yield a reliable output signal for the demodulator.